1. Technical Field
Embodiments of the invention relate generally to enhancing quality of images obtained from PET-MR scanners. Particular embodiments relate to attenuation correction of PET images.
2. Discussion of Art
Positron emission tomography (“PET”) machines use one or more rings of scintillators or other detectors to generate electrical signals from gamma rays (photon pairs) that produced from the recombination of electrons, within a target material, and positrons, emitted from decay of a radionuclide packaged in a tracer compound. Typically, recombination events occur within about 1 mm from the radionuclide decay event, and the recombination photons are emitted in generally opposite directions to arrive at different detectors. Paired photon arrivals that occur within a detection window (usually less than a few nanoseconds apart) are counted as indicating a recombination event, and, on this basis, computed tomography algorithms are applied to the scintillator position and detection data in order to locate the various recombination events, thereby producing three-dimensional images of the tracer disposition within the target material.
Typically, the target material is body tissue, the tracer compound is a liquid analogue to a biologic fluid, and the radionuclide is disposed primarily in body tissues that make use of the biologic fluid. For example, a common form of PET makes use of fluorodeoxyglucose (18F), which is analogous to glucose with the 18F radionuclide substituted for one of the hydroxyl groups ordinarily composing glucose. Brain matter, kidneys, and growing cells (e.g., metastasizing cancer cells) preferentially absorb both glucose and fluorodeoxyglucose. Therefore, PET is quite useful in oncologic studies, for localizing particular organs, and for studying metabolic processes.
One challenge in obtaining desired PET image quality is that gamma rays, in the energy spectrum produced by positron-electron interactions, are easily attenuated by typical body tissues and are differently attenuated by different body tissues. Varying attenuation will change the likelihood of detecting the recombination events from the patient, thereby confounding the process of making an image from the PET data. Accordingly, it is highly desirable to provide means for attenuation correction (“AC”).
For example, PET often is combined with computed tomography (“CT”), which uses a moving X-ray source and detectors to obtain images of internal structures. X-rays are photons and are attenuated much like the higher-energy photons produced from positron-electron recombination events. However, unlike PET photons, the source strength of CT photons is known. Thus, CT image data is a direct measurement of photon attenuation between source and detectors. Therefore, CT image data provides a useful basis for AC of concurrent PET imaging.
Combined PET-CT scans have proven particularly useful in respiratory-gated studies, for example studies undertaken during normal breathing in order to diagnose or evaluate lung cancer. Respiratory-gated images can reduce motion blurring that would otherwise result from acquisition of the PET image throughout a respiratory cycle. However, accurate reconstruction of respiratory-gated PET studies requires gate-specific attenuation correction information. The simultaneous acquisition of CT and PET data supports gate-specific AC.
For example, consider two elements of a PET detector where the line of response between the detectors passes through the lower thorax of the patient. At peak inspiration that line may pass through the lower lobes of the lung, which will attenuate the 511 keV photons by a relatively small amount. As the patient exhales the liver moves up into the thorax, so that at the end of expiration the same line of response may now pass through soft tissue, which will attenuate the 511 keV photons much more than the lung. If the PET data is gated so that there are different images being acquired at peak-inspiration and end-exhalation, each will benefit from an attenuation correction that corresponds to the actual distribution of attenuating tissue during that phase of the respiratory cycle.
Increasingly, and for a variety of reasons including lifetime radiation dose reduction goals, PET/CT scans are being replaced by combining PET scans with magnetic resonance imaging (“MR”). This new combination (“PET-MR”) presents novel technical issues. For example, whereas CT forms an image based on detection of X-rays emitted from a source through a target, MR forms an image based on detection of rotating “relaxation” magnetic fields that are produced within a target by nuclei that have odd atomic numbers, i.e., total number of neutrons and protons not divisible by two, in response to fluctuation of an imposed magnetic field. Thus, MR measures a phenomenon fundamentally different from the photons detected by CT and PET.
One advantage of MR is that magnetic fields do not attenuate in body tissues, so that nucleus location can be determined (using Fourier analysis) based solely on frequency shifting between the imposed magnetic field and the response field. Another advantage is that by careful selection of pulse sequence, distinct tissues or materials can be highlighted. Accordingly, MR frequently is used for differentiating tissue types within a patient, and also is used for identifying fine detail structures. Typically, different pulse sequences are used for tissue differentiation. For example, a T1 pulse sequence can be used to obtain images with water appearing darker and fat brighter. On the other hand, a T2 pulse sequence can be used to obtain an image with fat darker, and water lighter.
Thus, a single apparatus that combines PET and MR (a “PET-MR scanner”) can provide fine detail, tissue differentiation, and metabolic data. However, because MR signals do not attenuate in the same way as PET or CT signals attenuate, and because MR signal return is highly dependent on the type of pulse sequence used (with each pulse sequence emphasizing a different material), whereas the PET signal is attenuated by every layer of material intervening between a recombination event and a pair of detectors, single-scan MR image data does not necessarily provide a reliable basis for AC of concurrent PET imaging.
MR imaging has the capability to acquire a volume of images over the respiratory cycle, in a manner known as retrospectively gated cine-MRI. However, the types of MR images that can be acquired in a cine fashion have not been considered suitable for producing PET attenuation correction values. The MR protocols conventionally deemed effective for producing PET attenuation correction values, such as 2-point Dixon water-fat scans, typically require more than 10 seconds to acquire, thus, are not usable for capturing multiple images during a respiratory cycle. Instead, in order to minimize motion blur and other breathing artifacts, these MR image data typically have been acquired during a long breath-hold. However, a breath-held MR image does not show the intermediate lung positions that are needed for accurate AC of a PET scan.